1. Field of the Invention
The present invention relates generally to bio-electronic devices in which one or more films of an electronically active biochemical material are placed on a semiconductor substrate. More particularly, the present invention relates to such bio-electronic devices where the electronic activity of the biochemical films or layers is preserved through the use of one or more biochemical stabilization layers which are located between the semiconductor and biochemical films and between the biochemical films themselves.
2. Description of Related Art
The publications and other reference materials referred to herein to describe the background of the invention and to provide additional detail regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and grouped in the appended bibliography.
Semiconductors, materials that variably exhibit electrical conductivity, are used to fabricate solid-state electronic devices. Their utility arises from the property that their conductivity can change in response to environmental stimuli and because they are subject to photoelectric effects. Their properties arise from their crystalline nature, and become manifest at their surfaces.
Semiconductors are the product of a very creative and fortunate set of experiments performed in the 1940s by Shockley and Bardeen demonstrating the transistor effect (1). The consequences of their work has completely transformed the world over the latter half of the 20th century. Well recognized simple examples of semiconductor-based devices include the temperature-sensitive thermistor, makes use of the fact that semiconducting mixtures of certain metallic oxides decrease in electrical resistance with temperature increase. The device thus acts as a temperature sensor/switch, and enables a current to pass through that is proportional to the temperature change. Another semiconductor component is the light-sensitive photoconductive cell, or photoresistor. A photoresistor made from a thin film of cadmium sulfide has a resistance that can range from a high of millions of ohms in total darkness to a low of tens of ohms when it is illuminated with visible light. The device thus acts as a light sensor/switch and enables a current to pass through that is proportional to the, light change.
Diodes are devices that are based, in part, on the latter property of semiconductors, and those that have optoelectronic properties are especially important. Light-emitting diodes can be made from gallium arsenide, gallium phosphide, and certain other semiconductor compounds. These diodes emit a relatively narrow-frequency spectrum of optical radiation, which may range from the visible to the infrared, depending on the semiconductor from which the diode is made. Light-emitting diodes producing red, yellow, and green light have found widespread use as long-lived indicator lamps and as numeric displays in electronic calculators, wrist-watches, and other solid-state instruments. Infrared-emitting diodes are used for the optical transmission of information (optical communications) through highly transparent glass and plastic fibers, as well as through the atmosphere. The light-emitting diode laser is a junction diode that emits a narrow band of wavelengths of optical radiation when powered by an electric current above a specific threshold level.
Other kinds of optoelectronic diodes are used to detect optical radiation, some being specifically designed to detect various wavelengths and modulation frequencies. Solar cells are large-area junction diodes, most commonly made of doped silicon, that produce an electrical current in response to solar radiation having wavelengths ranging from the visible to the near infrared.
Other types of semiconductor devices include electronic switching devices such as silicon rectifiers, triacs and transistors.
The Materials Basis of Semiconductor Properties
One point that should be apparent from the above is that semiconductor materials are all essential crystalline ceramics. They may be metal oxide ceramics, intermetallic ceramics, or combinations thereof but tend to be comprised of elements in the central columns of the periodic table.
In chemical terms, what makes the semiconductor crystal so special is that the entire semiconductor crystal is a giant covalently bonded molecule. In semiconductors, electron wave functions are delocalized in principle over an entire macroscopic crystal. Because of the large spatial extent of these wave functions, no single atom can have much effect on the electron energies. Because semiconductors are drawn from the central columns (III, IV, V) of the periodic table and tend to be non-polar, electrons in both the valence and conduction bands (bonding and antibonding orbitals) tend to ignore the crystallographic lattice of atoms as well as one another. Therefore, instead of having one single chemical potential (or Fermi level) for all the electrons in the material, the possibility exists for two separate quasi-Fermi levels in the same crystal (2).
Semiconductor Surfaces are Critical to Their Function
Semiconductor surfaces are the most likely location for non-bonding or weakly bonding orbitals to occur. These non-bonding orbitals with unwanted energy levels in the forbidden gap may promote slow decay of electrons from the conduction band back into the valence band in a process that is known as "internal conversion ". This is also known as non-radiative recombination. Defect levels, arising from impurities within the crystal mass (also known as dislocations), act as stepping stones permitting conduction of electrons to cascade down to the valence band (2). For this reason, semiconductor processing requires exquisite control of the materials' surfaces and the cleanliness controls for manufacturing exceed both aerospace and medical standards (3).
Advances in semiconducting materials and related technologies enabling improved solid state electronics have arisen from the ability to grow macroscopically dislocation-free large diameter silicon single crystals, controlled growth of thick epitaxial layers, advances in processing techniques and an understanding of the interrelationship between "microstructures and device behavior". The techniques used to grow crystals are the following: a) Czochralski, b) float zone, c) liquid encapsulated Czochralski, and d) Bridgman. Certain devices, such as light-emitting diodes, double heterostructures or lasers, quantum well lasers, photodetectors and solar cells, all require multi layer structures consisting of layers of different compositions and conductivity. Among the various techniques used to deposit epitaxial layers or otherwise modify the surfaces of semiconductors include: chemical vapor deposition, organo-metallic vapor phase epitaxy, molecular beam epitaxy, organo-metallic molecular beam epitaxy, diffusion techniques and ion implantation techniques (4).
One of the common features of almost all semiconductor devices is that they exhibit relatively linear responsivities (5). For example, thermistors produce a linearly proportional increase in conductivity relative to an environmental temperature increase, photoresistors produce a linearly proportional increase in conductivity relative to environmental visible light radiation increase, and diodes become linearly more luminescent with an increase in current.
While linear responsivity is extremely useful in a broad range of applications such as biotechnology, physics, chemistry, medicine, aviation, oceanography and environmental control (5), ultra sensitive detection systems in all of these fields of use would have to exhibit non-linear transduction--would have to produce a disproportionately large signal in response to an extremely low level of substance, and a disproportionately small signal in response to an extremely high level of substance--to be maximally useful.
In particular, low threshold detection has been a major technological challenge. The task is not physically impossible, for there are ample examples of naturally occurring ultra-sensitive detection systems with a broad dynamic range. Indeed, biological systems are optimized for low level detection. Animals have optical and acoustic detection systems that are far more sensitive and have a far greater dynamic range than any manmade device. Insects and fish have extraordinary chemoreceptors far more sensitive than any synthetic device. Mimicry of these biological non-linear systems in a synthetic device, however, has only recently been explored.
Recent studies have shown that modifications in the surfaces of semiconductors can have a profound impact on their electrical performance. This is particularly true for polycrystalline semiconductor devices such as CdTe- and CulnSe.sub.2 based solar cells (7). Specifically, several recent studies have shown that exposure of semiconductors to organic ligands can change both semiconductor luminescence or flat band potentials (8, 9). In the primary study cited, semiconductor's electron affinity (surface conductivity) was modifiable over a 500 mV range with various substituted benzoic acid derivatives without affecting band bending. These findings suggest that semiconductors may function as chemoreceptors/sensors in a manner analogous to thermistors and photoresistors.
Organized Complex Molecules (Homodyads)
The rediscovery of surfaces has led to investigations in two directions. The first involves exploration of devices based exclusively on surface elements while the other involves exploration of bulk device tuning through surface modification. As an example of the former, efforts to create electronic functions and devices based on molecules instead of bulk semiconductors are being inspired by the anticipated enormous increase in computing speed and storage density. Among the challenges of coupling conjugated systems and confined aromatic systems with operational devices is to achieve charge transfer in low fields such as metallic wires and to establish communication with individual electrically separated nanometer structures or molecules. Conjugated polymers with mobile charge carriers in nanometer channels that exhibit significant conductivity when encapsulated have been described. The materials used include the mesoporous alumino-silicate host, designated MCM-41 and the conducting polymer, polyaniline (10). Organized complex molecules, known also as homodyads or dyads, have been fabricated from a wide variety of materials. Homodyads are simply paired electron donor molecules and electron acceptor molecules separated by a spacer molecule (11).
Organized Complex Electronically Active Molecules at Semiconductor Surfaces (Heterodyads)
Both the surface modification concept and the electron donor-acceptor pair concept can be combined at the semiconductor surface. Electron transfer across the space between a donor semiconductor and an acceptor organic molecule has been demonstrated in a three layered construct comprised of polycrystalline titanium dioxide, a salicylic acid related spacer molecule, and an electron acceptor from the bipyridinium family (12). The product was viewed as a regenerative photoelectrical cell based on transparent polycrystalline semiconductor films sensitized by chemisorbed dyes (13, 14). Many others have been involved in similar types of experiments.
Colvin et al. (15) describe a method for attaching semiconductor nanocrystals to metal surfaces using self assembled difunctional organic monolayers as bridge compounds. Recent advances have extended self assembled monolayers beyond the prototype gold/thiol systems. Fatty acids on aluminum, silanes on silicon, isonitriles on platinum and rigid phosphates on metals are all examples. Metals provide the ideal support for organic compounds with large non-linear optical behavior and by using self assembled monolayers, the molecules can be held in specific orientations with respect to the metal (16). In other work, the ability to dictate the structural details of an interface is exploited to study processes of electron transport between an electrode surface and an active moiety bound on top of a monolayer (17, 18, 19).
Yan and Bein demonstrate the potential of organically modified layered sorbents for the development of selective chemical sensors, for example, for aromatic compounds. The interplay of size exclusion and partitioning in the organic phases results in unique selectivities that can complement the molecular sieving of porous framework hosts such as zeolites. Shown is a four layer composition comprised of a silicate layer, an organic clay layer which is the molecular sieve and binding layer, an additional silicate and finally, a QCM gold electrode sensor (20).
Feng and Bein (12) reported the oriented growth of crystals of zinc-phosphate zeolite in gold surfaces modified with metal phosphonate multilayer films. The high degree of orientation observed is attributed to a strong affinity between the phosphonic acid groups of the phosphate multi layer and the (111) faces of the growing crystals. The systems described are the first examples of oriented surface controlled growth of molecular sieve crystals. These materials, they suggested could offer exciting applications such as controlled access of molecules of pre-selected size to a sensor surface or orientation of moleculars for non-linear optical applications.
In general, the very low light emission efficiency from the molecules adjacent to metallic and semiconductor surfaces is caused by the fast deexcitation processes governed by energy transfer to non-radiative surface excitations (Auger processes). Conductors such as indium-tin-oxides, which possess band structures that are responsible for the optical transparency in the visible and near ultraviolet and therefore preclude efficient energy transfer in this energy range become an ideal surface to measure light emission from immobilized molecules on their surfaces. Indium-tin-oxide surfaces were coated with high emission and photochemically stable molecules 9-10 dichloro anthracene and the laser dyes DCM and coumarin (21).
The design of selective coatings for microsensors such as optical waveguides, chemically sensitive field effect transistors, chemical resistors and acoustic wave devices has attracted growing attention. The goal of these studies is to increase the sensitivity and chemical selectivity of the sensor by controlling the surface interactions and solubilities of analyte vapors to be detected. Yan and Bein (22) use the coupling agent 3-mercaptopropyl-trimethoxysilane as a bifunctional molecular precursor for anchoring zeolite crystals to a gold electrode. A three layer composition comprised of a gold tin oxide quartz crystal microbalance coated with a cat ionic disilane coated with a zeolite arrangement has been used to control the self assembly of redox chains on electrodes (23).
Part of the problem of introducing non-linear functionality to semiconductor devices, essentially coupling allosteric electronically active biological molecules to the surfaces of semiconductors, arises from the molecular inactivation induced by the surfaces. It is important to note that not all molecules that are electron-active lose their activity following surface mobilization. Fumarate reductase from E. coli can be immobilized in an extremely electroactive state at an electrode with a retention of native catalytic properties. Fumarate serves as a terminal electron acceptor (24). In addition, many of the molecules which form homodyads may retain activity after being surface bound, although there is little data at present to support such a contention. More importantly, surface induced denaturation is well recognized. Loss of fluorescence activity after direct adsorption to solid surfaces, such as glass observed by Schlautman (25) using the dye perylene. The fluorescence of polycyclic aromatic hydrocarbons tends to be observed only with free solute and almost universally is quenched following adsorption or other association of non-organic materials. Fluorescence quenching of perylene approached almost 100% with the binding of perylene to humic acid as well, while bovine serum albumin quenched only 42% of the associated perylene fluorescence (25). In a similar vein, Gaines et al. have shown that a dyad may be stabilized by as much as 0.9 eV in going from a polar liquid to a rigid glass (26). Thus, solid states are clearly, in general, inhibitory of optimal electron carriage by electronically active biological molecules.
Surfaces also tend to constrain a molecule's ability to assume various allosteric conformations. This can negatively impact the performance of certain bio-opto/electronic devices. For example, optical memory devices, by virtue of the band width of laser devices, are expected to improve the density of microprocessor memory by a factor of 10. During the past eight years, investigators have shown considerable interest in using light-transducing proteins for optical memories. The use of biological molecules has significant advantages. The ability of biological molecules to respond to photons and convert to electrical energy in part, depends on changes in molecular shape. The shape change induces changes in a molecule's frequency response which could make a device based on shape-changing molecules self tuning as well as nonlinear (27). Restricting the allosteric activity by a non-yielding surface would preclude the realization of the projected gain.
Biosensors are another class of devices dependent on shape. Biosensors are analytical devices that respond selectively to analytes in an appropriate sample and convert their concentration into an electrical signal via a combination of a biological recognition system and a physico-chemical transducer. Biosensors promise to provide a powerful and inexpensive alternative to conventional analytic strategies for assaying chemical species in complex matrices; they do this by being able to discriminate the target analyte from a host of inert and potentially interfering species without the requirement for separating and, subsequently, identifying all the constituents of the sample. Specific fields of applications include veterinary medicine, agri-food, horticulture, pharmaceutics, petrochemical industry, environmental surveillance, defense and security (28). All biosensors exploit a close harmony between a selective biorecognition system and a transducer which translates a physico-chemical signal perturbation associated with the biorecognition process into a usable signal. The biorecognition system is typically an enzyme, sequence of enzymes, lectin, antibody, membrane receptor protein, organelle, bacterial, plant or animal cell or whole slice of plant or mammalian tissue. The majority of successful biosensors exploit enzymes as a biological recognition response system which are linked to transducers capable of responding to the protons, ions, gasses, heat, light, mass or electrons generated during the catalytic cycle. Biocatalytic systems based on enzymes can display poor stability, limited selectivity and insufficient sensitivity at low levels. Highly selective and sensitive devices based on immunological recognition systems may circumvent the shortcomings. One example of an antibody system included a surface acoustic wave quartz crystal which was comprised of interdigitated transducers between which was deposited a goat antibody by covalent immobilization to the silanized surface (29).
In general, the coupling of the semiconductor materials to biologically active molecules or even living tissues is extremely difficult, and the contact must be "gentle" (30). More technically, optimization of molecular response of nonlinear optical materials will depend crucially on how the chromophore molecule response is affected by environment and the temporal characteristics of that environment (31).